Light sources such as lasers that generate “ultra-short” light pulses are becoming more commonplace in high-tech industries as new methods are being developed to utilize their characteristics. “Ultra-fast” and “ultra-short” typically refer to the temporal duration (length) of a light pulse, and in particular to pulses having durations less than a few hundred femtoseconds (fs). Present-day lasers are capable of producing light pulses of 50-100 fs and as short as 6 fs. Future lasers will be capable of producing even shorter light pulses.
For such ultra-short pulses, a unique advantage is their extremely high energies over ultra-short time scales. Such pulses allow access to unique physical processes that only occur at these energies and time scales, such as laser micromachining and certain biological and medical applications (e.g., laser in-situ keratomileusis (LASIK)). Knowing the pulse irradiance (measured in watts per unit area) with a high degree of accuracy is critical for most processes that employ ultra-short light pulses. In order to quantify the irradiance of ultra-short light pulses, one must know their exact temporal width.
Because ultra-short light pulses exist for extremely short time periods by definition, there is no direct way to measure their width. This is largely because most atoms and materials do not react sufficiently fast. Accordingly, the state-of-the-art measurement techniques for ultra-short pulses rely on measuring a non-linear effect caused by an ultra-short pulse and then backing out the pulse length.
The most common technique for measuring ultra-short light pulses involves splitting the incident pulse (beam) inside a Michelson-style interferometer, with one interferometer arm sweeping back and forth. The distance the arm must sweep equates to the physical length of the laser pulse (e.g., a 100 fs pulse represents a length of 30 μm). The interferometer provides the autocorrelation of the pulse, which is measured using an oscilloscope. However, a Michelson interferometer is very sensitive to alignment so it typically takes significant time to set up and maintain it in proper operating condition for making measurements.
Another pulse measuring technique called Frequency Resolved Optical Gating (FROG) relies on splitting the incident beam into two separate beams and recombining the separated beams inside a non-linear crystal. Four-wave mixing occurs inside the crystal and a new beam is generated that has double the optical frequency of the input beam. The new beam is recorded via a detector (e.g., a charge-coupled device or “CCD”), which provides information about the frequency and temporal information of the new beam shape. When viewing the beam profile using the FROG technique, one axis represents the spectrum of the pulse (which is relatively wide since the pulse is relatively short), while the other axis represents the temporal shape. While the FROG technique is very convenient, it requires that the incident beam have a perfect Gaussian profile. Most lasers, however, do not have the requisite idealized Gaussian profile, so that the time and spectrum measurements tend to be inaccurate.
More recently, a pulse-width measuring technique was developed by Reid et al., and described in the article by Reid et al., entitled “Light-emitting diodes as measurement devices for femtosecond laser pulses,” Optics Letters, Vol. 22, No. 4, published on Feb. 15, 1997 (hereinafter, “the Reid article”), which article is incorporated by reference herein. The Reid article device utilizes a movable prism and an unbiased LED that has a non-linear power-dependent response. The prism splits an initial laser beam into two shifted beams that interfere. Moving the prism for each new incident pulse causes an autocorrelation interference pattern to sweep across the LED detector, which generates a corresponding autocorrelation signal.
Though various approaches to ultra-short pulse measurement are embodied in a number of different commercial devices, these devices have significant shortcomings. One serious shortcoming is that they are unusually difficult to use in practice mostly because they are difficult to align. This lack of functionality is particularly problematic given that frequent system alignment is needed for most light-pulse-measurement applications. Further, the measurement devices are subject to beam-shape limitations—that is to say, poor-quality beam shapes result in poor measurements. In addition, the typical ultra-short pulse measurement device is very costly—about $20,000 or more in present-day dollars.
Accordingly, efficient, cost-effective and commercially viable systems and methods for measuring ultra-short light pulses are needed.